In multi-wavelength optical communication systems (usually referred to as “wavelength division multiplexed” or WDM systems), efficient wavelength-selective means for tapping electromagnetic radiation (to be referred to herein as “light”, regardless of wavelength) from an optical fiber could be advantageously used in a variety of functions, e.g., as a wavelength monitor, channel monitor, demultiplexer, amplifier monitor, or in a feedback loop with an optical amplifier.
U.S. Pat. No. 5,061,032 issued to G. Meltz et al. on Oct. 29, 1991, discloses an optical fiber tap that comprises a blazed, chirped refractive index grating selected to redirect light guided in the fiber such that it comes to a focus at a point outside of the fiber. As used throughout this discussion, the term “blazed” refers to a grating where the plane of the index perturbations (i.e., gratings) is not perpendicular to the propagation direction of the guided mode or modes within the fiber. A grating is “chirped” if the (optical) repeat distance Λ between the index perturbations is not constant as a function of the axial coordinate z of the fiber, i.e., if Λ=Λ(z). An exemplary dispersive optical waveguide tap including a blazed and chirped refractive index grating is disclosed in U.S. Pat. No. 5,832,156 issued to T. A. Strasser et al. on Nov. 3, 1998. In particular, Strasser et al. utilize a coupling means disposed contiguous with the optical fiber in the region of the grating to direct the tapped modes into an associated detector apparatus, such as an array of photodiodes. The blazed grating in the fiber also functions to angularly (θ) disperse the light such that different wavelengths can be imaged by the coupling means onto different detector elements within the array. Consequently, the electrical signals of the detector array map out the spectrum of light associated with the original signal propagating through the fiber.
The optical performance and cost of various prior art optical monitors (as defined by parameters such as bandwidth, resolution and accuracy) are primarily determined by the grating/detector combination utilized. In many newer applications, such as measuring optical signal-to-noise ratios (OSNRs) of tightly-spaced dense WDM (“DWDM”) channels (e.g., 50 Ghz and closer spacings), extended dynamic range measurements, simultaneous monitoring of ultrawide (e.g., 60-80 nm) or multiple bands (e.g., C and L), as well as simultaneous monitoring of multiple network elements (e.g., amplifier and add/drop nodes), the combined cost/performance of the prior art devices does not meet the desired targets.
Thus, a need remains in the art for an optical waveguide tap that remains cost-effective while able to provide increased bandwidth and/or resolution for new and emerging optical applications.